1. Field of the Invention
The present invention generally relates to thermally processable, image-recording materials including silver halide. More particularly, the present invention is directed toward such materials which are capable of being thermally processed in the absence of water and/or base and which include a specified class of reductone developing agents.
2. Description of the Related Art
Thermally processable image-recording materials, including both thermographic (imaged and developed thermally) and photothermographic materials (imaged with light and developed thermally), are well known in the art. Such materials often include silver halide therein for forming images. In photothermographic systems, the silver halide is light sensitive, whereas in thermographic systems, the silver halide is typically light-insensitive (e.g. the silver halide may be de-sensitized with dyes as known in the art or may be processed under non-exposing lighting conditions, i.e. wavelengths of light which the silver halide is not sensitive to).
Thermographic materials typically comprise a polymeric support coated with a source of silver e.g. silver halide and/or a silver salt e.g. silver behenate, a binder, and a developing agent. Images are recorded and processed thermally; that is, images are formed by imagewise heating of the media which results in an imagewise reduction of silver, thus forming an image in metallic silver.
Photothermographic materials typically comprise a polymeric support coated with a light-sensitive silver halide emulsion and a developing agent. Images are recorded by exposing the light-sensitive silver halide to light, thereby forming a latent image. This latent image is subsequently reduced to a final image by the application of heat in the presence of the developing agent.
Photothermographic materials can be generally divided into two classes. The first class of materials utilize silver halide as the sole source of silver. That is, silver halide not only functions as light-sensitive material for forming a latent image, but also serves as the sole source of silver for forming a final image, e.g. the light-sensitive silver may be developed to form a final negative image in reduced silver (metallic silver). Materials of this sort typically include a polymeric support including in one or several layers: (a) a silver halide emulsion, (b) a developing agent for converting the exposed silver halide to metallic silver, (c) an alkaline activator to obtain a pH at which the silver halide can be effectively developed, and (d) a stabilizer to tie up any undeveloped silver halide. Similarly, silver diffusion transfer systems are known wherein unexposed silver halide is dissolved and transferred to a separate layer where it is subsequently reduced to form a positive final image in reduced silver, or reacts with a color-providing material to form a colored image.
The second class of photothermographic materials utilize light-sensitive silver halide for forming a latent image upon exposure, but unlike the first class of materials just described, this second class of materials also utilizes a non-light sensitive source of silver, i.e. a silver salt such as silver behenate, for forming a final image. With such materials, exposed silver halide, upon heating, catalyzes an oxidation-reduction reaction between the non-light sensitive silver salt and a developing agent to form a final image. Examples of such materials are disclosed in U.S. Pat. Nos. 3,751,255; 4,639,407 and 4,260,677 wherein images in reduced silver are formed by imagewise reduction of silver ions provided by a light insensitive silver salt.
Examples of color photothermographic system are disclosed in U.S. Ser. Nos. 923,843; 079,146; and 058,494; all assigned to the assignee of the subject invention. These references disclose thermally processed systems wherein a light insensitive silver salt is utilized as a source of silver ions made available imagewise, upon heating, to cleave a dye-providing material, thus releasing a diffusible dye species which forms a colored image.
For a more detailed explanation of thermographic and photothermographic materials, reference should be made to: D. H. Klosterboer in J. Sturge, V. Walworth, and A. Shepp, eds., Imaging Processes and Materials, Neblette's Eighth Edition, Van Nostrand Reinhold, New York, 1989, pp. 279-291.
As just described, thermographic and photothermographic materials utilize developing agents for reducing silver to form a final image. Often times conventional photographic developers will work in thermally processable systems; however, this is not always the case. The specific nature of the thermally processable material will dictate the operability of the developer. For example, if the thermally processable material includes base or base precursors, traditional photographic developing agents such as hydroquinones may often be utilized. However, if the pH of the system is insufficient to ionize substantial quantities of the hydroquinone, hydroquinone developing agents will not develop sufficient silver to be operable in the system.
Reductone developer agents are yet another class of developing agents commonly used in photographic systems, e.g. see U.S. Pat. Nos. 2,691,589; 3,615,440; 3,664,835; 3,672,896; 3,690,872; 3,816,137; and 4,371,603. Photothermographic materials may also include reductone developing agents, e.g. see U.S. Pat. Nos. 4,433,037; 4,550,071; and 4,639,407. Although generally operable in photographic systems, reductone developer agents are often times inoperable in thermally processed systems, particularly those systems which are "dry" and base-free, i.e. those thermographic or photothermographic systems which are substantially free of water and base and which are thermally processed in the absence of water or base, as described in detail hereinbelow.